With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the fabrication process of the multilayer interconnects with finer circuit, as the number of interconnect levels increases, film coverage (or step coverage) of step geometry is lowered in thin film formation because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnects, it is necessary to improve the step coverage and planarize the surface. It is also necessary to planarize semiconductor device surfaces so that irregularity steps formed thereon fall within a depth of focus in optical lithography. This is because finer optical lithography entails shallower depth of focus.
Accordingly, the planarization of the semiconductor device surfaces is becoming more important in the fabrication process of the semiconductor devices. Chemical mechanical polishing (CMP) is the most important technique in the surface planarization. This chemical mechanical polishing is a process of polishing a wafer by bringing a wafer into sliding contact with a polishing surface of a polishing pad while supplying a polishing liquid containing abrasive grains, such as silica (SiO2), onto the polishing surface.
FIG. 22 is a schematic view of a polishing apparatus for performing CMP. This polishing apparatus includes a polishing table 203 for supporting a polishing pad 202, a polishing head 201 for holding a wafer W, and a polishing liquid supply nozzle 205 for supplying a polishing liquid (or slurry) onto the polishing pad 202. The polishing pad 202 is rotated together with the polishing table 203, while the polishing liquid is supplied onto the rotating polishing pad 202. The polishing head 201 holds the wafer W and presses the wafer W against a polishing surface 202a of the polishing pad 202 at predetermined pressure. A surface of the wafer W is polished by a mechanical action of abrasive grains contained in the polishing liquid and a chemical action of chemical components contained in the polishing liquid.
If a relative pressing force applied between the wafer W and the polishing surface 202a of the polishing pad 202 is not uniform over the entire surface of the wafer W during polishing, the surface of the wafer W is polished insufficiently or excessively in different regions thereof, which depends on pressing force applied thereto. It has been customary to uniformize the pressing force applied to the wafer W by providing a pressure chamber formed by an elastic membrane at a lower portion of the polishing head 201 and supplying the pressure chamber with a fluid, such as air, to press the wafer W under a fluid pressure through the elastic membrane.
The polishing pad 202 is so elastic that pressing forces applied to an edge portion (or a peripheral portion) of the wafer W become non-uniform during polishing, and hence only the edge portion of the wafer W may excessively be polished, which is referred to as “edge rounding”. In order to prevent such edge rounding, a retainer ring 220 for holding the edge portion of the wafer W is provided so as to be vertically movable with respect to a head body to thereby press the polishing surface 202a of the polishing pad 202 in an area around the peripheral portion of the wafer W.
Since the retainer ring 220 presses the polishing pad 202 in an area around the wafer W, a load of the retainer ring 220 affects a profile of the edge portion of the wafer W. In order to positively control a profile of the edge portion of the wafer W, a local load may be applied to a part of the retainer ring 220. The polishing apparatus shown in FIG. 22 is provided with a local-load exerting device 230 for exerting a local load on a part of the retainer ring 220. This local-load exerting device 230 is secured to a head arm 216.
FIG. 23 is a perspective view of the local-load exerting device 230 and the polishing head 201. As shown in FIG. 23, a stationary ring 235 is disposed on the retainer ring 220. The local-load exerting device 230 has a push rod 231 for transmitting a downward load to the retainer ring 220. The lower end of the push rod 231 is secured to the stationary ring 235. While the retainer ring 220 rotates during polishing of the wafer W, the stationary ring 235 and the local-load exerting device 230 do not rotate. The stationary ring 235 has the below-described rollers which make rolling contact with the upper surface of the retainer ring 220. The local-load exerting device 230 transmits a downward local load from the push rod 231 to the retainer ring 220 through the stationary ring 235.
FIG. 24 is a diagram, as viewed from above the retainer ring 220, of a mechanism for applying the local load to a part of the retainer ring 220. As shown in FIG. 24, a circular rail 221 is fixed to an upper surface of the retainer ring 220, and three rollers 225 are disposed on the circular rail 221. An annular groove 221a is formed in an upper surface of the circular rail 221, and the rollers 225 are placed in this annular groove 221a. 
FIG. 25 is a perspective view of the circular rail 221 and the rollers 225 disposed on it. The depiction of the retainer ring 220 has been omitted from FIG. 25. One of the three rollers 225 is coupled to the local-load exerting device 230 and, as shown in FIG. 25, a downward local load is exerted on this roller 225. The circular rail 221 rotates together with the retainer ring 220 during polishing of a wafer, while the three rollers 225 are each kept in a fixed position. Accordingly, these rollers 225 make rolling contact with the rotating circular rail 221.
When the circular rail 221 is rotating together with the retainer ring 220, there is a difference in speed between an inner side and an outer side of each roller 225 because the circular rail 221 has an annular shape as a whole. Accordingly, each roller 225 slips slightly due to the difference in speed. Further, when the circular rail 221 is rotating, the side surfaces of each roller 225 make contact with the annular groove 221a of the circular rail 221. Due to such slippage and contact of the rollers 225, the rollers 225 wear and thereby may generate wear particles. Moreover, the rollers 225 can break as their wear progresses. If the wear particles fall on the polishing pad, such wear particles may scratch the surface of the wafer during polishing of the wafer, thus causing a defect in the wafer.
The rotating retainer ring 220 may tilt due to manufacturing accuracy and surface irregularities of the polishing pad 202. Since the push rod 231 is secured to the stationary ring 235, the push rod 231 also tilts as the retainer ring 220 tilts. When the push rod 231 tilts, an excessive frictional resistance may be generated in a linear guide (not shown) that supports the push rod 231, resulting in a failure to apply an intended local load to the retainer ring 220. This may result in a failure to obtain a desired polishing result, and may cause a variation in thickness of a film especially in the peripheral portion of the wafer W.
Further, the local-load exerting device 230 may be slightly inclined with respect to the retainer ring 220 upon fixing of the local-load exerting device 230 to the head arm 216. If the local-load exerting device 230 itself is inclined with respect to the retainer ring 220, a stress is applied to the push rod 231 in a direction other than the vertical direction, whereby an excessive frictional resistance is generated in the above-described linear guide (not shown). This may also result in a failure to obtain a desired polishing result, and may cause a variation in thickness of a film especially in the peripheral portion of the wafer W.
In addition, when the polishing table 203 is rotating, the surface of the polishing table 203 may fluctuate up and down. Such a fluctuation of the polishing table 203 in the vertical directions may cause the entire retainer ring 220 to vibrate vertically. The local-load exerting device 230, which has its frictional resistance and large inertia, cannot absorb the vibration of the retainer ring 220, and as a result, the local load on the retainer ring 220 may also fluctuate.